Flexography is a method of printing that is commonly used for high-volume runs. Flexography is employed for printing on a variety of substrates such as paper, paperboard stock, corrugated board, films, foils and laminates. Newspapers and grocery bags are prominent examples. Coarse surfaces and stretch films can be economically printed only by means of flexography. Flexographic printing plates are relief plates with image elements raised above open areas. Such plates offer a number of advantages to the printer, based chiefly on their durability and the ease with which they can be made.
Although photopolymer printing elements are typically used in “flat” sheet form, there are particular applications and advantages to using the printing element in a continuous cylindrical form, as a continuous in-the-round (CITR) photopolymer sleeve. CITR photopolymer sleeves add the benefits of digital imaging, accurate registration, fast mounting, and no plate lift to the flexographic printing process. CITR sleeves have applications in the flexographic printing of continuous designs such as in wallpaper, decoration and gift-wrapping paper, and other continuous designs such as tablecloths, etc. CITR sleeves enable flexographic printing to be more competitive with gravure and offset printing processes on print quality.
A typical flexographic printing plate as delivered by its manufacturer, is a multi-layered article made of, in order, a backing or support layer, one or more unexposed photocurable layers, a protective layer or slip film, and a cover sheet. A typical CITR photopolymer sleeve generally comprises a sleeve carrier (support layer) and at least one unexposed photocurable layer on top of the support layer.
It is highly desirable in the flexographic prepress printing industry to eliminate the need for chemical processing of printing elements in developing relief images, in order to go from plate to press more quickly. Processes have been developed whereby photopolymer printing plates are prepared using heat and the differential melting temperature between cured and uncured photopolymer is used to develop the latent image. The basic parameters of this process are known, as described in U.S. Pat. Nos. 5,279,697, 5,175,072 and 3,264,103, in published U.S. patent publication Nos. US 2003/0180655, and U.S. 2003/0211423, and in WO 01/88615, WO 01/18604, and EP 1239329, the teachings of each of which are incorporated herein by reference in their entirety. These processes allow for the elimination of development solvents and the lengthy plate drying times needed to remove the solvent. The speed and efficiency of the process allow for use in the manufacture of flexographic plates for printing newspapers and other publications where quick turnaround times and high productivity are important.
The photopolymer layer allows for the creation of the desired image and provides a printing surface. The photopolymers used generally contain binders, monomers, photoinitiators, and other performance additives. The composition of the photopolymer should be such that there exists a substantial difference in the melt temperature between the cured and uncured polymer. It is precisely this difference that allows the creation of an image in the photopolymer when heated. The uncured photopolymer (i.e., the portions of the photopolymer not contacted with actinic radiation) will melt or substantially soften while the cured photopolymer will remain solid and intact at the temperature chosen. Thus the difference in melt temperature allows the uncured photopolymer to be selectively removed, thereby creating an image.
The printing element is then selectively exposed to actinic radiation. Preferably, the photopolymer layer is coated with an actinic radiation (substantially) opaque layer, which is also sensitive to laser ablation. A laser is then used to ablate selected areas of the actinic radiation opaque layer creating an in situ negative, and the printing element is then flood exposed through the in situ negative.
Once the photopolymer layer of the printing element has been selectively exposed to actinic radiation, it can then be developed using heat. As such, the printing element is generally heated to at least about 70° C. The exact temperature will depend upon the properties of the particular photopolymer being used. However, two primary factors should be considered in determining the development temperature:                1. The development temperature is preferably set between the melt temperature of the uncured photopolymer on the low end and the melt temperature of the cured photopolymer on the upper end. This will allow selective removal of the photopolymer, thereby creating the image.        2. The higher the development temperature, the quicker the process time will be. However, the development temperature should not be so high as to exceed the melt temperature of the cured photopolymer or so high that it will degrade the cured photopolymer. The temperature should be sufficient to melt or substantially soften the uncured photopolymer thereby allowing it to be removed.        
Once the heated printing element has been developed, uncured photopolymer can be melted or removed. In most instances, the heated printing element is contacted with a material that will absorb or otherwise remove the softened or melted uncured photopolymer. This removal process is generally referred to as “blotting”. Blotting is typically accomplished using a screen mesh or an absorbent fabric. Either woven or non-woven fabric is used and the fabric can be polymer based or paper, so long as the fabric can withstand the operating temperatures involved. In most instances, blotting is accomplished using rollers to bring the material and the heated printing plate element into contact.
U.S. Pat. No. 5,175,072 to Martens, the subject matter of which is herein incorporated by reference in its entirety, describes the removal of uncured portions of the photopolymer by using an absorbent sheet material. The uncured photopolymer layer is heated by conduction, convection, or other heating method to a temperature sufficient to effect melting. By maintaining more or less intimate contact of the absorbent sheet material with the photocurable layer, a transfer of the uncured photopolymer from the photopolymer layer to the absorbent sheet material takes place. While still in the heated condition, the absorbent sheet material is separated from the cured photopolymer layer in contact with the support layer to reveal the relief structure. After cooling, the resulting flexographic printing plate can be mounted on a printing plate cylinder.
Upon completion of the blotting process, the printing plate element is preferably post-exposed to further actinic radiation in the same machine, cooled and then ready for use.
Current thermal development apparatuses using heated rolls for blotting away the uncured photopolymer typically use only one heated roll that is of approximately the same width as the plate. This increases the difficulty in making printing elements of different sizes. In addition, additional problems may arise when attempting to make the blotting machine larger to accommodate larger printing elements.
Another problem with the current blotting methods is that a tremendous amount of force (approximately 100 pounds/linear inch) must be applied by the heated roll to force the blotting material into the image on the printing element. This large force can cause the heated roll to bend, resulting in an uneven floor. Furthermore, the heating and blotting process must often be repeated several times in order to obtain effective removal of the uncured photopolymer.
In addition, the biggest downfall to the commercialization of thermally produced plates is relief variation. Due to the fact that not all of the uncured resin may get processed, the amount of resin that actually gets removed can be affected by various conditions in the process. For instance, the type of image being processed can affect relief tolerance, such as combination copy, in which screens and solids are both present within the same image. Next, the actual set up conditions in the thermal processor, such as the number of cycles and the processing temperature, can affect uncured resin removal in a non-uniform manner. Resin removal, via a thermal process, is a very empirical-based output, and is ultimately dependent on the true need of the end-user. Thus, it is highly likely that the final process will result in residual un-cured resin remaining on the plate that will then become cured during the resultant post-exposure steps.
A further complication of thermally processing plates is that the traditional process of producing seamless photopolymer sleeves can inherently produce floor variations. While flat plates that are processed thermally may have floor inconsistencies that are manifested from the thermal process itself, the true floor of such plates is actually very uniform. This uniformity can be confirmed by simply processing the plate in a solvent-based developing system. However, for printing sleeves, the light exposure techniques that exist today are limited by the ability to expose the sleeve through the back side of the sleeve carrier, which ultimately results in an inherent floor variation. This inherent variation can be further complicated by the thermal process itself, which defines the need for an improved method of eliminating this pitfall in the sleeve manufacturing process.
To that end, the present invention is directed to an improved method of manufacturing photosensitive printing elements that minimizes relief variation and improves image fidelity. By providing a fully cured layer underneath the image layer, a sleeve can be properly processed without concern over floor variation or loss in image fidelity.
Furthermore, exposing, developing and post exposure/detack steps have traditionally been carried out in separate devices. This requires additional time to transfer the printing element from the exposure device to the development device and can affect the quality of the finished plate as a result of handling the printing element. Thus, it would be desirable to accomplish the exposing, developing and post exposure/detack steps in the same apparatus in order to improve both the quality and the accuracy of the final product.